An analysis of the ability of human spermatozoa to fertilize will be made on a large population of fertile and suspected infertile males. The fertilizability assay is an in vitro test utilizing the penetration of zona-free hamster eggs as the criteria for fertilization. Swelling human sperm heads in the cytoplasm of these eggs is taken as morphological evidence of the sperm's fertilizability. The assay procedure will be expanded to use both mouse and rat eggs to allow more widespread use of such a fertilization assay in routine semen analysis. Freezing techniques for the eggs will be developed to allow the development of a fertility test package which could be used in hospitals without access to a hamster, rat, or mouse colony. The in vitro fertilization procedure will be coupled with a Y body determination to ascertain the fertilization ratio of X to Y sperm in a given semen sample.